1. Field of the Invention
The invention is directed to thermoplastic blends of aromatic carbonate polymers, polybutylene terephthalate, styrene-acrylonitrile copolymer, and a methacrylate-butadiene-styrene shell-core copolmer.
2. Brief Description of the Prior Art
Aromatic carbonate polymers, such as polycarbonates, have become major commercial thermoplastics in view of their excellent physical properties. Furthermore, the physical properties can sometimes be improved by blending with other thermoplastics, although the choice of the type of additive thermoplastic and the amount is often quite critical.
For example, impact strength of carbonate polymers can be improved by blending with appropriate quantities of selected impact modifiers, such as core-shell copolymers of acrylonitrile-styrene copolymerized and grafted onto a core of butadiene-styrene elastomer or onto a core of an acrylic elastomer; see for example Liu, U.S. Pat. No. 4,503,183 (General Electric Co., 1985).
Another property of polycarbonates which can be improved by blending with additives is processability. For example, blends of polycarbonates with styrene-acrylonitrile copolymers have significantly improved flow properties, even when fiber-reinforced as taught by Van Abeleen in U.S. Pat. No. 4,393,161 (General Electric Co., 1983).
Useful blends of carbonate polymers with polybutylene terephthalate are also well known and commercially available. For example, General Electric's resin blend is marketed under the Trademark XENOY .RTM.. The main properties imparted to a base resin of polycarbonate imparted by the polybutylene terephthalate added to the polycarbonate are solvent resistance, environmental stress failure resistance, and lower melt flow viscosity. Such blends of carbonate polymers with polybutylene terephthalate can also be further improved by making three-component blends with various impact modifiers added such as methacrylate-butadiene-styrene and like shell-core copolymers (General Electric Co., Eur. Pat. Appl. 105,388[1982]).
However, despite the many useful carbonate resin blends that have been found, many troublesome constraints, limitations, and pitfalls are found in the blending of carbonate polymers. With few exceptions, most high molecular weight polymers do not form true solutions (single phase mixtures) with one another. The improved properties of most blends are instead associated with the formation of microdomains or phases of differing polymer content. The physical separation of the blend components to form such phases frequently leads to delamination and weak weldlines in objects molded from the polymer blends. Weldlines, also known as "knit lines" are the surfaces of juncture between the flows coming into the mold cavity from different gates; it is important that such weldlines should not constitute weak places in molded articles made by use of multigate (such as double gate) molds.
Another problem encountered with many otherwise useful carbonate polymer blends is poor flow properties during molding. To facilitate molding, the rheology of the polymer blend should exhibit the desirable property of shear thinning, i.e. reduction of viscosity with increased rate of shear. Shear thinning permits faster molding cycles at a given molding temperature; or, at a given temperature, shear thinning permits complete filling of the mold with lower pressure requirements.
A further problem encountered with carbonate polymer blends has been a tendency towards excess embrittlement (decline in impact strength) at low temperatures.
All three requirements, namely the requirement for strong weld lines, the requirement for shear thinning properties, and the requirement for good low temperature properties, are especially important for large molded articles such as automotive instrument panels. All three requirements have hitherto been difficult to simultaneously satisfy in polycarbonate blends containing styrene-acrylonitrile (SAN) impact modifiers, in polycarbonate-polybutylene terephthalate blends, or in polycarbonate-polybutylene terephthalate-impact modified blends. It is commonly found that as one property is improved, one or both of the other desired properties suffers, and the compromise may be generally unsatisfactory.
I have now found, that articles molded from certain four-component blends of carbonate polymer base resins have, simultaneously, good high and low temperature impact, good shear thinning properties, and strong weldlines. Such blends are achieved by using critical amounts of four blending components.